This invention is broadly applicable to linking a large battery pack, as in an Electric Vehicle (EV), with an existing grid-tied inverter, as in a PhotoVoltaic (PV) solar system, to provide bidirectional ancillary services such as frequency regulation to an electric utility grid operator. The resulting service revenue can provide an incentive for EV and solar PV ownership. Management of one-way power flow to electric vehicles has been described as in U.S. Pat. No. 8,278,881, Woody (assigned to GM) and from electric vehicles for emergency power as in U.S. Pat. No. 7,582,979, Oyobe (assigned to Toyota). The objective here is bidirectional power flow, both in and out, under remote control.
Electric vehicles, because of their large and expensive battery packs are more expensive than equivalent Internal Combustion (IC) powered vehicles. At the same time these expensive vehicles are typically parked overnight and not used. Similarly, PV systems with expensive grid-tied inverters which convert Direct Current (DC) power from the PV panels to Alternating Current (AC) power and provide it to local loads and the grid are idle after sundown at night. This invention has as its objective to enable both of these underutilized assets to be linked and used to perform useful ancillary services to the grid and to generate revenue from the electric grid Independent System Operator (ISO) as described by Beck (2009). It is a further object of this invention to provide a method which is broadly applicable to virtually any electric vehicle and any type of grid-tied inverter. This method and the associated Electric Vehicle-PhotoVoltaic (EVPV) linking apparatus are unlike the Electric Vehicle Equipment for Grid-Integrated Vehicles described in U.S. Pat. No. 8,509,976, Kempton (2013). Kempton's patent and related patent applications rely on a specialized Electric Vehicle Equipment (EVE) on board the vehicle to communicate with the stationary Electric Vehicle Service Equipment (EVSE) which charges the vehicle battery pack and links the vehicle to the grid for ancillary service revenue. Kempton further relies on power electronic equipment onboard the vehicle to perform the high current charging and DC-AC inversion required for frequency regulation. The present invention relies on an off-board EVPV and an existing off-board inverter for both of these functions.
The SEGIS-ES equipment described in detail in U.S. Pat. No. 8,463,449, Sanders, (2013), which is an outcome of a similarly titled research program at Sandia National Laboratory, is similar in concept to the present invention. Bidirectional chargers are available form Princeton Power Systems and Dynapower. The unique feature of the present invention is the linkage of virtually any existing battery pack as in Electric Vehicles and virtually any existing grid-tied inverters as in solar PhotoVoltaic systems to achieve a minimum-cost electric storage capability to support the grid and earn revenue as an incentive for EV and PV ownership.
It is not possible to link the battery pack directly to the inverter because PV inverters typically use Maximum Power Point Tracking (MPPT) technology to optimize the power drawn from the solar PV array as the solar irradiance fluctuates during the day. A PV array is limited in both voltage and current and MPPT technology varies the impedance of the inverter searching for the “knee” of the current-voltage curve where the PV power output is maximized. A battery pack has a limited voltage but almost unlimited current capability, and an MPPT inverter will increase power without limit until something fails. Furthermore, ancillary service requests are proportional, not on-off, so a means of controlling the power output of the battery pack is required. A DC-DC converter can perform both current limiting and power control functions.
To obtain revenue from the ISO it is necessary to provide power flow in either direction on request, and this can be accommodated by provision of a battery charger with an equivalent output to the inverter and the DC-DC converter. In principle this charger could also charge the battery, but there are practical considerations that rule against this. Most EVs have on-board chargers which are specifically designed and controlled to charge their batteries safely. Since one of the objectives of this invention is to obtain ancillary service revenue for any vehicle with a DC quick charge port without modification of the vehicle or tampering with its onboard systems, an independent off-board charger is indicated.
Another reason for having matched DC-DC converters and off board chargers is that the vehicle battery can deliver more power than is normally used in charging it, and ancillary services are reimbursed on the basis of symmetric power available for both up and down regulation. Typically vehicles are charged at either the J1772 level 1 rate from a 120 V AC source at 15 A or 1.9 kW or the level 2 rate from a 240 V AC source at 24 or 40 Amps (5.8 or 9.6 kW). However a 24 kWh battery as in the popular Nissan Leaf battery electric vehicle can deliver power at 24 kW the 1.0 C rate (complete discharge in one hour) or even more, and get paid for it at that rate, provided that the charge rate is the same. Traction batteries in the mid range of their state of charge are designed for heavy drains for acceleration and heavy charging currents for regenerative braking.
Fortunately, modern battery chargers which are switching power supplies can also double as DC-DC converters, so that the same type of equipment can provide both services.
The inverter must be capable of providing power to the grid (a so-called grid-tied inverter) as used in most solar PV installations. The incentive for this is that the grid can provide power to the user at night when the sun isn't shining and the PV array can feed excess power to the grid during the day. The utility and economics of solar PV depend on this interchange of energy, since a stand alone PV system with enough battery storage to cover local loads over an extensive period of bad weather in the winter would be exorbitantly expensive. Most solar PV installations benefit from an agreement with their local utility known as net metering in which a bidirectional electric meter measures the power flow in and out, but the customer is billed only for the net input. This clearly is critical for ancillary service revenue, because if the customer is charged for the power input but not credited with the reverse flow, the energy bill far outweighs the ancillary service power revenue.
Grid-tied inverters have extensive electronic controls to synchronize their output with the grid. They also have so-called anti-islanding features, so that if there is a power outage, the solar PV power is turned off to prevent it from being fed back into the part of the grid that is still connected to the solar location and posing a hazard to the linemen sent to fix the outage. This loss of solar power just when it would be most useful is an aggravating feature of existing PV systems. In principle this invention can alleviate this problem by providing power through the inverter even at night during an outage by disconnecting the solar system from the grid and spoofing the inverter to start up again. A safer alternative is to provide a separate inverter serving only local loads and switching the DC connection to it during outages. A less costly alternative is posed by some of the newer inverters which have an internally switched receptacle that can be powered by the solar PV array for limited amounts of emergency power during an outage. Here the back up feature of this invention to keep the emergency power flowing at night or in bad weather from the EV battery pack could be very valuable.
Inverters are relatively short-lived relative to PV arrays and the balance of the PV system. In some cases it will pay to anticipate replacement and install a larger inverter to accommodate the high power capability of this invention to earn more service revenue from a higher power capability of the whole system and to get the emergency power feature mentioned above.
The primary reason that inverters fail is that the inlet filter capacitors experience too much voltage variation due to a significant 60 Hz AC ripple on the DC output of the PV array due to the AC load imposed by the inverter. A necessary feature of this invention is a very large input capacitor to the inverter which will minimize this AC ripple and should increase the life of the inverter.